There are several major television networks providing national news and other programming to local or regional affiliate stations. These affiliate stations receive a National Television Systems Commission (NTSC) network feed via, e.g., a satellite link and synchronize the NTSC feed to a local NTSC synchronizing, or "genlock" signal. The term "genlocking" refers to the process of synchronizing one or more signals, or the equipment producing the signals, to a studio timing reference. The NTSC television signal includes synchronizing pulses which delineate horizontal lines of video and, therefore, allow for simple calculations of video field and frame locations within the NTSC signal. Thus, the affiliate is able to align the video and audio information on, e.g., a line by line, field by field, or frame by frame basis. The affiliate station may then easily insert ("splice in") advertising or local programming into the received signal. The resultant spliced signal, which includes network programming and locally inserted material, is then transmitted via, e.g., terrestrial broadcast.
In several communications systems, the data to be transmitted is compressed so that the available bandwidth is used more efficiently. For example, the Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172. The second, known as MPEG-2, refers to ISO/IEC standards 13818.
The proposed use of such compressed data by a network and its affiliate poses several technical challenges since MPEG-based video and audio synchronization is not as straightforward as NTSC-based synchronization. Moreover, the affiliate station must be able to subject an MPEG data stream to operations like genlock or splicing (e.g., adding commercials to programs) prior to re-transmission by the affiliate station.
There is considerable expense involved in performing the above synchronization, splicing and other operations on a compressed (e.g., MPEG) data stream. This is because the compressed network feed or other stream must be decoded, processed and re-encoded prior to transmission to a local user. In addition to the expense, this approach also degrades the signal quality of the network fed video and audio because the steps of decoding and encoding are not entirely lossless (i.e., portions of the data representing an image may be ignored or truncated during at least the encoding process).
Therefore, a need exists in the art for a cost-efficient method and apparatus for applying splicing and other processing techniques to information contained in a compressed data stream.